You can archive accidentally created task from an experiment, but you cannot delete them.
To archive a task, first, open an experiment to enter its canvas. Then click on Edit experiment button just above the canvas on the left. Select the task and click on the arrow in the right upper corner to open a drop-down menu (see the image below).
Select Archive task from the listed actions and the task will be moved to archive. After that don't forget to click the green Save button.